Such axial plunging units serve the transmission of torque in shaft assemblies which, when in operation, are subject to changes in length. These changes in length in a motor vehicle, for example, are caused by movements of one of the attachment parts due to compression and rebound actions. To the extent that reference is made to shaft assemblies, it is also possible for the latter to be formed largely by the longitudinal plunging unit itself which is then directly connected to attachment pieces such as welded-on flanges and slipped-on hubs.
When a profiled sleeve is displaced relative to a profiled journal while under torque, the rolling movement of the balls in the inner and outer ball grooves is substantially free from any sliding friction, so that the balls and thus the ball cage with all the balls cover half the relative plunging distance between the two parts. This permits the change in length of the longitudinal plunging unit to be of a low-friction nature. At the ends of the plunging path, axial stops for the ball cage or for the balls have to be provided in the profiled sleeve or on the profiled journal. Under normal operating conditions, the ball cage should preferably not reach the axial stops, but it should be freely displaceable on the plunging path specified by the design, thus avoiding any sliding friction of the balls in the grooves.
Assemblies of this type are used in torque-transmitting shafts, especially driveshafts which—for compensating for tolerances in the distance between the attachment parts during assembly and/or for compensating changes in the distances between the attachment parts, such as joints, in operation—require low-friction plunging conditions. When in operation and during the transmission of torque, such driveshafts suffer from a problem in that the inter-engaging portions of the profiled sleeve and of the profiled journal are, by necessity, subject to torsion in the regions of the ball grooves.
As a result of the torsional movements of the profiled sleeve and of the profiled journal during the transmission of torque, there can occur uneven load conditions at the balls in the different cross-sectional planes. This adversely affects smooth and easy movability and can lead to early failure. In order to avoid this, it is desirable for both parts to feature a high degree of stiffness.